Halogenated solvents are used by a wide range of industries including dry cleaners, electronic equipment manufacturers, metal parts fabricators, insecticide and herbicide producers, military equipment manufacturers, etc. These solvents replaced petroleum derived mineral spirits and have distinct advantages because of their nonflammability. The persistence and mobility of these hydrocarbons in the subsurface was largely unanticipated, therefore historical disposal practices have led to widespread groundwater contamination. For example, trichloroethylene (TCE) has been found at more than 791 of 1300 National Priority List sites, primarily as a groundwater contaminant.
Chlorinated solvents fall into the category of dense non-aqueous phase liquids (DNAPLs). DNAPLs are heavier than water and therefore sink below the groundwater table until they encounter a layer through which they cannot pass. As they move downwards, DNAPLs leave behind a smearing trace of their migration pathway before eventually pooling on a confining unit or perhaps within a crevice of a fractured rock. Most DNAPLs can dissolve in aqueous environments, yet they do so in such small quantities that the original contaminant pool functions as a subsurface contamination source. The portion of the contaminant that does dissolve is typically at concentrations which exceed allowable groundwater standards.
Treatment of halogenated hydrocarbon contaminated groundwater is usually accomplished by pumping the groundwater to the surface and removing the contaminant through oxidation or air stripping. Pump-and-treat remediation systems have experienced limited success with respect to DNAPLs. Capillary pressure holds DNAPLs at residual saturation which can represent significant contamination. Consequently, removal of the contaminant from the subsurface is extremely time consuming, and therefore cleanup goals are rarely achieved. However, even though the pump-and-treat method is not a particularly successful remediation technology, it has proven to be a highly efficient tool for containment of the contaminant.
Because of the limited degree of success in remediating contaminated sites with technologies which attempt to remove the contaminant from the subsurface and pump it to a treatment system, recent efforts have focused on the physical, biological, or chemical treatment of these contaminants in situ.
A permeable treatment wall (PTW) is an alternative remediation technology which does not require groundwater to be pumped to a treatment facility. Instead contaminated groundwater is passively treated in situ. Permeable treatment walls, as shown in FIG. 1, are vertical cells which are installed subsurface near a contaminant source. PTWs are designed to have a greater permeability than the surrounding soils, and are typically constructed using a high permeability sand mixture comprising a zero-valent metal. PTWs have been successfully demonstrated in several field studies and offer potential economic savings over other halogenated solvent treatment methods.
It has been shown that zero-valent zinc and iron significantly enhanced the reductive dehalogenation of aliphatic compounds with iron being particularly attractive due to its low cost and availability. Batch tests in which aqueous solutions of a wide range of chlorinated methanes, ethanes, and ethenes were added to 100-mesh iron filings resulted in degradation rates that were three to seven orders of magnitude greater than natural abiotic rates reported in the literature. Generally, the rates increased with the degree of chlorination and with increasing iron surface area to solution ratio. The chlorinated products of degradation subsequently degraded to non-chlorinated compounds. Similar results have been obtained by Vogan et al. who propose that the corrosion of iron, while occurring independently of volatile organic compound degradation, likely provides the electron source needed for the reduction (Vogan, J. L. et al. "Evaluation of In Situ Groundwater Remediation by Metal Enhanced Reductive-Dehalogenation--Laboratory Column Studies and Groundwater Flow Modeling," presented at the 87th Annual Meeting and Exhibition of the Air Waste Management Association, Cincinnati, Ohio, June 19-24).
Although PTWs appear to represent a promising technology for renumerating contaminated groundwater, over time, researchers have noted that the effectiveness of many PTWs often decreases. This decrease in effectiveness may result from corrosion of the metal reagent utilized in the PTW, which reduces the amount of metal surface area available to participate in the reductive decontamination reaction. Alternatively, this decrease may result from the formation of particulate contaminates in or around the PTWs, which reduce the flow of contaminated water through the PTW. The term "particulate fouling" is used herein to describe this formation of particulate contaminates. Accordingly, there is a current need for methods to increase the effectiveness of PTW's which are functioning at sub-optimal levels as a result of corrosion on the metal reagent or as a result of particulate fouling.